N-(3-Methyl-2-pyridyl)-3,4,5,6-tetrachlorophthalmic acid and N-(6-methyl-2-pyridyl)-3,4,5,6-tetrachlorophthalmic acid are known to be pharamacologically active having been shown to exhibit a hypertensive effect in biological systems (Dolzhenko et al., 2003). In the context of this study, these materials are of interest for their potential as a UV-active dye for dye-sensitized solar-cell applications. Heating these compounds to 333 K in hydrated methanol produces crystals of two salts, namely 2-amino-6-methylpyridin-1-ium 2-carboxy-3,4,5,6-tetrachlorobenzoate, (I) (Fig. 1), and bis(2-amino-3-methylpyridin-1-ium) 3,4,5,6-tetrachlorophthalate-3,4,5,6-tetrachlorophthalic acid (1/1), (II) (Fig. 2). These salts are the result of the reaction of the starting material with water present in the methanol solution and the equilibrium that exists between amides and water and the corresponding amines and carboxylic acids. In (I) and (II), protonation of the pyridyl N atom results in pyridinium salts stabilized by imino resonance. Interestingly, as 3,4,5,6-tetrachlorophthalic acid cocrystallizes with 2-amino-3-methylpyridin-1-ium 3,4,5,6-tetrachlorophthalate all the products of this reaction are represented stoichiometrically in the crystal structure.
The molecular geometry of the 2-amino-6-methylpyridin-1-ium cation in the structure of (I) (Table 1) can be compared with that of the nonhalogenated 2-amino-6-methylpyridinium 2-formylbenzoate monohydrate (Büyükgüngör & Odabasoglu, 2006). This geometry is similar in both compounds, with the characteristic bond-length alternation within the pyridyl ring, which demonstrates the imino resonance stabilizing the positive charge (Fig. 3) (Zhi et al., 2002). The bond geometry of the aromatic ring in the 2-carboxy-3,4,5,6-tetrachlorobenzoate anion in (I) resembles more closely that of the hemihydrate of 3,4,5,6-tetrachlorophthalic acid (Ito et al., 1975) than that of the 2-carboxy-3,4,5,6-tetrachlorobenzoate in a similar salt, 2-methyl-5-ethylpyridinium 3,4,5,6-tetrachlorophthalate (Galloy et al., 1976). This indicates a contribution from the neutral canonical form, which is also observed in the carboxylate group, where O3-C8 is observed to be shorter than O4-C8. The bond distances in the aromatic ring of 2-carboxy-3,4,5,6-tetrachlorobenzoate in (I) range from 1.374 (9) to 1.403 (9) Å.
Compound (I) forms continuous sheets of hydrogen-bonded ions parallel to (010) (Table 2). These sheets contain the characteristic rings having graph set R22(8) (Etter, 1990; Bernstein et al., 1995) with the amine and pyridinium N atoms acting as donors and the two carboxylate O atoms acting as acceptors (N1-H1NO4 and N2-H2BO3), as is well documented in this type of compound (Quah et al., 2010; Hemamalini & Fun, 2010). These rings are linked by chain motifs to form the sheets. The 2-carboxy-3,4,5,6-tetrachlorobenzoate anions form chains parallel to the  direction through O1-H1O4ii interactions to give a graph-set motif of C(7) (symmetry codes as in Table 2). The 2-amino-6-methylpyridin-1-ium cations link via the anions forming chains with the graph sets C22(9) (through N2-H2BO3 and N2-H2AO2i) and C22(11) (through N1-H1NO4 and N2-H2AO2i), giving an overall C22(9)C22(11)[R22(8)] chain of rings parallel to the  direction (Fig. 5).
The aromatic rings in the 2-amino-3-methylpyridin-1-ium cations of compound (II) exhibit similar geometry to those in (I) with regard to the bond distances (Table 3). The distances in the benzene rings of the 3,4,5,6-tetrachlorophthalate dianion and the 3,4,5,6-tetrachlorophthalic acid molecule are in the ranges 1.387 (3)-1.403 (2) and 1.389 (3)-1.399 (3) Å, respectively, altogether more consistent than the bond distances of the 2-carboxy-3,4,5,6-tetrachlorobenzoate anion in (I) or the hemihydrate of 3,4,5,6-tetrachlorophthalic acid. This similarity in bond geometry between the dianion and the neutral acid in (II) strengthens the argument for the contribution of the neutral canonical forms in these compounds. As is observed in similar structures (Ni et al., 2007; Zhi et al., 2002), there are - interactions between 2-amino-3-methylpyridin-1-ium cations; the dihedral angle between the two pyridinium rings in the selected asymmetric unit is only 3.8 (2)° and the corresponding centroid-centroid separation is 3.834 (2) Å.
Unlike the two-dimensional network observed in (I), the hydrogen-bonded system in (II) consists of a finite array of four 2-amino-3-methylpyridin-1-ium cations, two 3,4,5,6-tetrachlorophthalate dianions and two molecules of 3,4,5,6-tetrachlorophthalic acid (Fig. 6 and Table 4). The R22(8) ring motifs formed between the pyridinium and phthalate ions are once again present, with each phthalate dianion forming two such rings parallel to each other because of the - interactions between the pyridinium cations, i.e. through N2-H2AO1 and N1-H1NO2 for one ring, and N4-H4AO3 and N3-H3NO4 for the other. In addition to this, each of these rings is connected to an adjacent R22(8) ring through N2-H2BO3i and N4-H4BO1i forming an [R22(8)R42(8)R22(8)] motif within an outer R44(16) ring (symmetry code is as in Table 4). Further motifs are observed when considering that there are two parallel [R22(8)R42(8)R22(8)] motifs linked by the phthalate dianions; this gives rise to rings with graph sets R44(18) and R44(22).
The 3,4,5,6-tetrachlorophthalic acid molecules and 3,4,5,6-tetrachlorophthalate dianions are also connected by hydrogen bonds with the acid protons donating to carboxylate O-atom acceptors through O5-H5OO2 and O7-H7OO4 to create R22(14) motifs. Though there is no direct hydrogen bonding between the acid molecules and the pyridinium cations, rings with the graph set R86(34) are formed between acid molecules through the [R22(8)R42(8)R22(8)] motif.
| Figure 1 |
The structure of the asymmetric unit of (I), with atomic displacement ellipsoids drawn at the 50% probability level.
| Figure 2 |
The structure of the asymmetric unit of (II), with atomic displacement ellipsoids drawn at the 50% probability level.
| Figure 3 |
The resonance exhibited by (I).
| Figure 4 |
View of the C(7) hydrogen-bonding motif in the  direction in (I). H atoms not involved in hydrogen bonding (dashed lines) have been omitted.
| Figure 5 |
View of the C22(9)C22(11)[R22(8)] chain of rings along  in (I). H atoms not involved in hydrogen bonding (dashed lines) have been omitted.
| Figure 6 |
Stereoview of the finite hydrogen-bonding network in the structure of (II). H atoms not involved in hydrogen bonding (dashed lines) have been omitted.
N-(3-Methyl-2-pyridyl)-3,4,5,6-tetrachlorophthalmic acid (10 mg, 0.026 mmol) and N-(6-methyl-2-pyridyl)-3,4,5,6-tetrachlorophthalmic acid (10 mg, 0.026 mmol) were heated to 333 K in hydrated methanol (5 ml) until a clear solution was obtained. Colourless plate-like crystals of (I) were grown upon cooling to room temperature and colourless prism-like crystals of (II) grew after the solution was allowed to stand for one week.
Mr = 412.04
Orthorhombic, P c a 21
a = 9.441 (14) Å
b = 12.56 (2) Å
c = 13.69 (2) Å
V = 1623 (4) Å3
Z = 4
Mo K radiation
= 0.75 mm-1
T = 120 K
0.33 × 0.16 × 0.06 mm
Rigaku Saturn724+ (2 × 2 bin mode) diffractometer
Absorption correction: multi-scan (ABSCOR; Higashi, 1995) Tmin = 0.866, Tmax = 0.956
5083 measured reflections
2495 independent reflections
2302 reflections with I > 2(I)
Rint = 0.048
R[F2 > 2(F2)] = 0.057
wR(F2) = 0.127
S = 1.10
H-atom parameters constrained
max = 0.44 e Å-3
min = -0.50 e Å-3
Absolute structure: Flack (1983)
Flack parameter: -0.31 (13)
|C7-O2 ||1.205 (7) |
|C7-O1 ||1.413 (8) |
|C8-O3 ||1.248 (8) |
|C8-O4 ||1.344 (8) |
|C9-N1 ||1.361 (9) |
|C9-N2 ||1.402 (10) |
|C13-N1 ||1.427 (10) |
|O2-C7-O1 ||127.0 (6) |
|O3-C8-O4 ||134.0 (5) |
|N1-C9-N2 ||124.9 (6) |
|C9-N1-C13 ||129.2 (5) |
|D-HA ||D-H ||HA ||DA ||D-HA |
|N2-H2BO3 ||0.86 ||1.81 ||2.666 (8) ||173 |
|N2-H2AO2i ||0.86 ||2.11 ||2.931 (8) ||159 |
|N1-H1NO4 ||0.86 ||1.71 ||2.559 (7) ||171 |
|O1-H1OO4ii ||0.82 ||1.83 ||2.604 (7) ||158 |
|Symmetry codes: (i) ; (ii) . |
H atoms were positioned geometrically and refined as riding on their parent atoms, with C-H = 0.95 Å and Uiso(H) = 1.2Ueq(C), and N-H = 0.88 Å and Uiso(H) = 1.2Ueq(N). Hydroxy and methyl H atoms were modelled in a similar fashion, with O-H = 0.84 Å and Uiso(H) = 1.5Ueq(C), and C-H = 0.98 Å and Uiso(H) = 1.5Ueq(N). The most disagreeable reflections were omitted and those exhibiting a (F2) value greater than 5 s.u. were removed; 5 from (I) and 31 from (II). The refinement was further improved by restricting the reflections considered to those with 25.68°. The Flack parameter for (I) gives the expected values for a correct absolute structure within 3 s.u. Nonetheless since the s.u. is moderate, the inverted structure was tested. This yielded a Flack parameter of x = 1.21 (13) by the `hole-in-one' method and of x = 1.34 (13) using TWIN/BASF, giving us confidence that we have presented the correct absolute structure with respect to the polar-axis direction. These checks were particularly important given that the precision of the Flack x parameter is poor owing to a low Friedel coverage of 60%. Refinement for (II) was limited to those reflections with < 25.68° reducing the number of missing data; however, a number of missing data remain (201 reflections between min and sin/ = 0.600). Analysis of reciprocal-space plots reveal that these missing portions are fairly randomly dispersed which gives us confidence that this is not a systematic error. Moreover, the missing data were comprised of high-angle reflections that were just outside the reach of the data collection strategy.
For both compounds, data collection: CrystalClear (Rigaku, 2008); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: WinGX (Farrugia, 1999).
Supplementary data for this paper are available from the IUCr electronic archives (Reference: GD3389 ). Services for accessing these data are described at the back of the journal.
JMC thanks the Royal Society for a University Research Fellowship, the University of New Brunswick for the UNB Vice-Chancellor's Research Chair (JMC), and NSERC for Discovery Grant No. 355708 (for PGW).
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